It is well known that silver halide photographic emulsions are produced by spectral sensitization. It is also well known that three or more sensitizing dyes are used to effect spectral sensitization for light in the visible region.
The intensity of spectral sensitization depends on the chemical structure of the sensitizing dyes used, and on the properties of the emulsion such as the composition of the silver halide, its crystal habit, crystal form, as well as the concentrations of silver ion and hydrogen ion. The spectral sensitivity also depends on the type of the photographic addenda incorporated in the emulsion, such as stabilizer, antifoggant, coating aid, precipitating agent and color coupler. Another factor which determines spectral sensitivity is the composition of the developer including the developing agent, alkali concentration or solvent for the silver halide.
If the amount of the sensitizing dye is increased, the intensity of spectral sensitization increases accordingly, but if the dye is used in an excess amount, the sensitivity of the silver halide for the characteristic absorption region (i.e., blue sensitivity) is decreased. This decrease in the sensitivity of silver halide for the characteristic absorption region by the sensitizing dye causes a decrease in the white sensitivity, which is not desirable as relates to photographic properties.
These defects are particularly undesirable for photographic materials produced from emulsions sensitized orthopanchromatically (e.g., black-and-white negative taking materials, micronegative materials and photographic materials for the silver salt diffusion transfer process). As described in Japanese Patent Publication No. 42493/73 and other prior art references, the distribution of spectral sensitivities of taking materials sensitized orthopanchromatically has serious effects on the improvement of tone reproduction.
The desired distribution curve of spectral sensitivities is such that the red sensitivity is maximum in the range of 625 to 640 nm, with sensitivity for the longer wavelengths extending on a gentle slope to about 650 nm.